It is a well known fact that in scanning optical systems of printers, scanners and the like the scanner lens absorbs moisture and the focal point in the vertical scanning direction moves in such a way that the focal length increases (for example, Patent Document 1). Such a movement of the focal point due to moisture absorption is generated by a change in refractive index inside the scanner lens caused by moisture absorption. Accordingly, a method in which an appropriate allowance is added to width in the vertical scanning direction of the scanner lens has been proposed (for example, Patent Document 1).
However, conventionally, an allowance of width in the vertical scanning direction of the scanner lens has been determined by shape of the scanner lens alone, and characteristics of the scanning optical system have not been taken into account. In other words, a scanner lens and a method for producing it in which an allowance of width in the vertical scanning direction of the scanner lens is determined in consideration of characteristics of the scanning optical system to be used, have not been developed.    Patent Document 1: JP5-303010A
Accordingly, there is a need for a scanner lens and a method for producing it in which an allowance of width in the vertical scanning direction of the scanner lens is determined in consideration of characteristics of the scanning optical system to be used.